we talk about vibrations, something may be bobbing up and down, or a pendulum swinging back and forth, or something that occurs and recurs. i could take my chalk on the board-- i can vibrate it up and down. i'm vibrating a little bit, too, okay? these are vibration, all right? now, how frequently i go to and fro has to do with frequency. we talk about frequency with the letter f. here's a low frequency. wanna see a high frequency? this is common-sense stuff. see that? ain't that neat? guess what a very high frequency is. how many say, "gee, i don't know what--" come on, you guys know. what's a very high frequency? [makes sounds] like that, yeah? okay. but when then a high frequency is propagated through space, watch. it traces out what we call a... sine curve. it's really a sine curve but a representation of a wave, okay? now, we gonna go through some definitions, gang. this frequency dragged out. the dotted line is my equilibrium point. i'm gonna vibrate above the equilibrium and below, and i go just as far above and just as far as below. i call this particular distance here, by the